The present invention relates to methods for cyclone boiler flame diagnostics and control. More particularly, the present invention provides methods for monitoring the operating state of a cyclone furnace using linear and nonlinear signal analysis techniques, including temporal irreversibility and symbol sequence. Adjustments may be made in the air flow distribution to optimize performance. Economic pressures and increasingly restrictive environmental regulations have contributed to an increasing need for advanced control systems that efficiently regulate utility boilers. Inefficient boiler control is responsible for wasting large amounts of fuel and releasing nitrogen oxide pollutants into the atmosphere.
In particular, industrial and utility cyclone boiler operators need a better indication of the overall combustion condition in the cyclone furnace. A cyclone furnace consists of two combustion cavities—the burner and the barrel. There are three common types of burners—radial, scroll and vortex. Although the arrangements differ slightly, the basic principles of operation for each type are the same. In the burner, primary air and tertiary air are combined with crushed coal to ignite the coal. The purpose of the primary air is to ignite the coal. The purpose of the tertiary air is to cool the burner face and provide axial momentum to force the combustion products out of the burner and into the barrel. The partially combusted coal and combustion gases pass into the barrel where secondary air is added. Combustion air in the burner and the barrel is introduced tangentially to create a swirling flow of air and gas that forces the burning coal to the walls of the combustion chambers. In the barrel, a recirculation pattern of air and combustion gases is created in the outer annulus of the cavity from the re-entrant throat end of the barrel back toward the burner, and then back through the barrel in a center vortex exiting the barrel through the re-entrant throat. In the burner, the temperature of the coal particles and residual ash is below the ash melting temperature, so the particles of coal and ash pass out of the burner without sticking to the walls of the burner. In the barrel however, the temperature of the coal and ash reaches the ash melting temperature. A pool of slag (melted ash) forms on the bottom and sides of the barrel. This molten slag serves to hold the large coal particles in place while combustion of the coal particle is completed. It is essential for the efficient operation of the cyclone for this slag layer to remain in a molten state. The product gases and a small amount of ash and unburned coal char pass out of the barrel through the re-entrant throat and into the main boiler cavity where combustion is completed. The molten slag is continuously removed from the cyclone barrel through a spout at the re-entrant throat known as a slag tap.
Historically, the cyclonic flow in the cyclone furnace has led to very high combustion temperatures, which, although providing very efficient combustion, has also led to high NOx emissions. The high NOx emissions were due to both the nitrogen in the fuel (fuel NOx) and thermal or prompt NOx from the dissociation of nitrogen in the air and oxidation to NO. Industry's response to this problem was to stage the combustion in the cyclone. This was achieved by providing less than the stoichiometric amount of air required to completely burn the coal, and providing the balance of the air required to complete combustion through additional ports in the boiler known as over-fire air ports. This has two beneficial impacts on NOx emissions. First, in the absence of sufficient oxygen, the fuel nitrogen will recombine with itself to form molecular nitrogen rather than NO. Second, since less fuel is burned in the barrel, the operating temperature is lower, and therefore, the thermal NOx is reduced.
Although NOx emissions have been reduced considerably with this technique it has led to two significant operational problems. First, under reducing conditions (i.e., stoichiometries less than 1.0 molar ratio of air/fuel) corrosion and wastage of the refractory walls of the cyclone barrel is accelerated. Second, lower temperatures in the cyclone barrel may cause the temperature of the molten slag to fall below its melting temperature and freeze. If the slag freezes, the burning coal particles that exit the burner will not adhere to the walls of the barrel and will be ejected from the cyclone before the combustion can be completed. This leads to high unburned carbon losses from the boiler and lower boiler efficiency. If the slag tap opening is plugged with frozen slag, the cyclone may have to be shut down to remove the slag manually. An additional complicating factor is that the melting temperature of the slag is a strong function of the properties of the coal. Since utility and industrial boiler owners buy coal with a wide range of properties to reduce the operating cost of the boiler, the melting temperature of the slag can vary considerably day-to-day. Further, other characteristics of the coal, such as moisture and grind size can negatively impact combustion in the cyclone.
Currently, operators must make visual observations of each cyclone to monitor the condition of slag in the barrel and the quality of combustion. This is a qualitative inspection, and varies from operator to operator. If the operator observes that the slag is beginning to freeze, the lighter, which is typically only used for startup, may be used to increase the temperature of the slag and melt the slag layer. Since the lighter typically uses a premium fuel such as natural gas or oil, this is an expensive corrective action. Also, by increasing the operating temperature of the barrel, the NOx emissions may also increase. Some units are equipped with secondary air dampers that are split into two or three sections. The operator can adjust the position of these dampers to redistribute the secondary air along the length of the barrel to concentrate more air in the vicinity where the slag is freezing. This action increases combustion in the vicinity of the frozen slag and consequently raises the temperature of the slag. This requires frequent visual inspections on the part of the operator to closely follow the situation.
Periodically, it is necessary for the operator to tune the cyclones to optimize the performance or to accommodate a change in the cyclone (e.g., coal properties, hardware changes, refractory replacement, etc.). The procedure for tuning the cyclones to optimize performance is iterative and time-consuming. Temporary, expensive gas sample grids must be installed in the flue at the exhaust of the boiler to measure the concentration of carbon monoxide and unburned carbon in the combustion gases. From these measurements, the boiler operator must infer changes that need to be made to the air distribution on a trial-and-error basis to improve performance.
A monitoring technique is therefore needed that can assess the quality of combustion in the burner and barrel, and provide guidance to the operator to make adjustments in the air distribution to maintain optimum performance without resorting to expensive gas grids. In this invention as described below, the techniques are proven to be helpful in providing the cyclone furnace operators with a better indication of the cyclone operating status. Further, a method to control the air distribution within cyclones using the analysis results is described. Improving the air distribution and reducing the cyclone down time are important benefits of this invention.
The flame physics in a cyclone barrel differ significantly from the physics in a wall-fired burner. There are two non-obvious elements to the invention. First, prior to the discovery contained in this application it was not known whether or not it would be possible to correlate the flame characteristics with excess air, air distribution, NOx emissions, slag tap operation or some other aspects of cyclone operation. Although the means have been developed to reliably acquire the flame scanner signals and analyze those signals in real time, the analysis techniques in U.S. Pat. Nos. 6,775,645 B2 and 6,901,351 B2 were developed for pulverized coal combustion rather than cyclone furnaces. Prior to this invention it was not clear what issues may need to be addressed to extend the technology to cyclones. The primary effort in the development program was to modify the existing algorithms, add new analysis techniques and correlate the analysis results with specific combustion conditions in the cyclone. Second, it was not known if a method could be developed to adjust the air flow distribution within the cyclone using the analysis results to optimize the cyclone performance.
Systems that can accurately reflect combustion conditions are essential to advanced boiler management. In the case of low-NOx burners, accurate monitoring of burner-operating states is more important than for conventional burners because low-NOx burners are more sensitive to changes in operating parameters. Conventional combustion monitoring systems provide information that is averaged over many burners and long time scales (e.g., measurements of excess air, coal feed, or NOx emissions at time scales of several minutes or hours). However, large fluctuations in NOx emissions and carbon burnout can occur in individual burners over short time scales (i.e., between about 10 seconds to fractions of a second). These fluctuations produce widely different boiler performance for operating conditions that are otherwise indistinguishable. Accordingly, combustion diagnostics should reflect both long and short time-scale transients for more reliable boiler optimization. Cyclone furnaces contain added complexity that demands a more sophisticated combustion monitoring approach. Since the combustion is divided into two zones as described above, the distribution of primary and secondary air must be carefully controlled to achieve the desired degree of combustion.
A significant advantage of cyclone furnaces over wall-fired burners is that the ash (incombustible portion of fuel) is trapped in the cyclone barrel and leaves the boiler via a slag tap at the discharge end of the cyclone. It is very important that the slag be maintained in a free flowing state. If the slag freezes, the combustion in the cyclone will deteriorate and an unacceptable amount of the fuel may leave the cyclone unburned. The air flow distribution within the cyclone must be carefully controlled to maintain the slag in a free-flowing state as well as to achieve the desired degree of combustion.
A key variable in the combustion of fossil fuels, such as oil, gas and pulverized coal, is the air/fuel (“A/F”) ratio. The A/F ratio strongly influences the efficiency of fuel usage and the emissions produced during the combustion process (especially, for low-NOx burners). Generally, lower A/F ratios produce lower NOx emissions. However, carbon monoxide and unburned carbon emissions may increase if the A/F ratio is too low. The A/F ratio also affects slagging, fouling and corrosion phenomena that typically occur in the combustion zone. In current steam generators fired with fossil fuel, the A/F ratio is controlled by measurement of oxygen and/or carbon monoxide (“CO”) concentration in the stack gases or at the economizer outlet. In either case, the gas measurement is taken at a location removed from the actual location of the combustion process. Unfortunately, in multi-burner or multi-cyclone steam generator furnaces the A/F ratio may differ from burner to burner, or cyclone to cyclone. Since both combustion efficiency and NOx generation levels depend on the localized values of the A/F ratio (i.e., the distribution and mixing within each flame), measurement and control of the global A/F ratio alone does not necessarily optimize performance. Further, a minimum A/F ratio must be maintained in the cyclone to provide sufficient heat to maintain the slag in a free-flowing state.
In a cyclone a number of factors can alter the optimum A/F ratio during normal boiler operation. These variables include changes in fuel moisture or heating value, changes in fuel blend, changes in coal size distribution due to wear in the crusher or changes in coal grinding properties, changes in the overall air flow rate, and changes in the distribution of air among individual burners or cyclone barrels. All cyclone burners (especially with staged air and/or fuel injection) undergo characteristic transitions in dynamic stability (i.e., bifurcations) as the above parameters are varied. In the cyclone burners the most important burner bifurcations are caused by the nonlinear dependence of flame speed on the relative amounts of fuel and air present. In particular, flame speed (i.e., combustion rate) drops exponentially to zero when the A/F ratio approaches either fuel-lean or fuel-rich flammability limits. Fuel-lean refers to conditions where excess air (i.e., oxygen) is present and fuel-rich refers to conditions where excess fuel is present. Local variation in the A/F ratio creates some zones within the burner that sustain combustion and other zones that do not sustain combustion. These zones may interact through complex mechanisms that depend on the details of turbulent mixing imposed by burner design, specific operating settings, and the relative amounts and spatial distribution of incoming fuel and air. The complexity of the process is further increased by the presence of both solids and volatile components in the fuel, which mix and burn at characteristically different rates. The details of the distribution and interaction of combusting and non-combusting zones is critical in determining the efficiency of fuel conversion and the levels of pollutants emitted (such as oxides of nitrogen and carbon monoxide).
Extinction of combustion within the burner or along the length of the barrel represents a bifurcation in which the flame state is no longer stable. Generally, the primary air flow is minimized to keep coal in the burner long enough to initiate combustion. Extinction of combustion in the burner can occur if the primary air flow is too high, if there is an excessive amount of moisture present in the fuel or if the fuel is too coarse or a combination of all three. These changes in properties cause a delay in the release of volatile matter from the fuel resulting in a gas mixture outside of the flammability limit. Whether caused by high air velocity or excessively fuel-rich burner conditions, delayed combustion in the burner is an undesirable operating condition typically associated with excessive emissions of pollutants.
Currently, there is no good way to identify that the optimum amount of primary air is being fed to the burner. Knowing the air flow alone is not sufficient. It is also necessary to have a measure of the quality of combustion in the burner. Further, the optimum amount may change as the properties of the coal change, therefore requiring a means to continuously monitor the combustion in the burner and to provide guidance to the operator to make adjustments. U.S. Pat. Nos. 6,775,645 B2 and 6,901,351 B2 describe the methods to calculate time asymmetry (temporal irreversibility) and symbol sequence histograms from flame scanner signals.
Conventional signal analysis methods such as Fourier analysis and univariate statistics are based on assumptions that are not entirely valid for combustion in a cyclone burner or barrel. Specifically, Fourier analysis assumes that the described processes are linear (i.e., processes in which the observed behavior is produced by superposition of simple modes), while univariate statistics assumes that each event is random and independent from events at other times, i.e., there is no time correlation. When these assumptions are incorrect the results from Fourier analysis and univariate statistics can provide either misleading results or results that are insensitive to real differences (M. J. Khesin et al., “Demonstration Tests of New Burner Diagnostic System on a 650 MW Coal-Fired Utility Boiler,” American Power Conference, Chicago, Ill., Volume 59-1, 1997; Krueger et al., “Illinois Power's On-Line Operator Advisory System to Control NOx and Improve Boiler Efficiency: An Update,” American Power Conference, Chicago, Ill., Volume 59-1, 1997; Adamson, et. al., “Boiler Flame Monitoring Systems for Low NOx Applications—An Update,” American Power Conference, Chicago, Ill., Volume 59-1, 1997; Khesin, M., et al., “Application of a Flame Spectra Analyzer for Burner Balancing,” presented at the 6th International ISA POWID/EPRI Controls and Instrumentation Conference, June 1996, Baltimore, Md.)
Chaos theory (especially, symbol sequence techniques and temporal irreversibility) avoids the assumptions of conventional analytical methods and thus may provide information unavailable from these well-known techniques. Chaos theory is a prominent new approach for understanding and analyzing deterministic nonlinear processes, which provides specific tools for detecting and characterizing fluctuating unstable patterns of these processes (Gleick, “Chaos: Making a New Science,” Viking Press, New York, 1987; Stewart, “Does God Play Dice? The Mathematics of Chaos,” Basil Blackwell Inc., New York, 1989; Strogatz, “Nonlinear Dynamics and Chaos,” Addison-Wesley Publishing Company, Reading, Mass., 1994; Ott et al., “Coping with Chaos,” John Wiley & Sons, Inc., New York, 1994; Abarbanel, “Analysis of Observed Chaotic Data,” Springer, N.Y., 1996). Chaos theory has been applied to feedback systems and burner flame analysis (Wang et al. U.S. Pat. No. 5,404,298; Jeffers, U.S. Pat. No. 5,465,219; Fuller et al., “Enhancing Burner Diagnostics and Control with Chaos-Based Signal Analysis Techniques,” 1996 International Mechanical Engineering Congress and Exposition, Atlanta, Ga., vol. 4, pp 281-291, Nov. 17-22, 1996; J. B. Green, Jr. et al., “Time Irreversibility and Comparison of Cyclic-Variability Models,” Society of Automotive Engineers Technical Paper No. 1999-01-0221 (1999)). Because combustion is highly nonlinear, analytical techniques derived from chaos theory (especially, symbol sequence techniques and temporal irreversibility) may be particularly useful for burner flame analysis.
Thus, it has become apparent that a new method for monitoring the operating state of cyclone burners and barrels is needed. In particular, what is needed is a method that can monitor the operating states of individual burner and barrels using nonlinear analytical methods such as symbol sequence analysis and temporal irreversibility on a diagnostically meaningful time scale.
B&W and UT-Battelle (ORNL) completed a feasibility test, two baseline test series, and a demonstration test at the Ameren UE Sioux Plant that is located in West Alton, Mo. The inventors also completed two baseline test series and a demonstration test at the Alliant Energy Edgewater Plant located in Sheboygan, Wis. Data were collected at nominal operating conditions and some upset conditions. The dynamic content of the signals were analyzed using analysis techniques described in U.S. Pat. Nos. 6,775,645 B2 and 6,901,351 B2. Nonlinear statistics such as time asymmetry and symbol sequence patterns responded to changes in stoichiometry and primary/secondary/tertiary air split. In addition, it was determined that higher statistical moments of the flame scanner signals such as skewness and kurtosis strongly correlate with the symbol sequence patterns and time asymmetry in flame signals. Thus skewness and kurtosis can be used to supplement the information from the more advanced nonlinear features. The demonstration tests showed that the claimed method for adjusting air flow as guided by the analysis results effectively optimized the cyclone operation.